High-frequency noise is one of the causes for IC data errors. Among known methods for removing such high-frequency noise, one method includes provision of a capacitor having high capacitance on a wiring board. Such a capacitor of high capacitance can be provided by forming a layer of high dielectric constant on the wiring board. A material of high dielectric constant is useful for miniaturization of a built-in antenna or thinning of a radio wave absorber, since the size of the antenna or the thickness of the absorber is almost inversely proportional to the square root of dielectric constant. Particularly, demand has arisen for a resin material exhibiting excellent workability or moldability and having high dielectric constant.
Meanwhile, wireless data communication requires an antenna. Particularly, a non-contact IC card/tag including no power supply—in which electromagnetic wave energy radiated from a reader/writer is converted into drive power for a built-in IC chip—requires performance enhancement and miniaturization of an antenna.
Such high performance of antenna wiring boards may be attained through approaches depending on the frequency range employed for wireless communication. In a well-known method, a loop-like wiring pattern (serving as a coil) and electrodes (corresponding to electrodes of a capacitor) are provided on a wiring board for an antenna, to thereby form a tuning circuit suitable for a frequency employed for wireless communication. In this method, a capacitor of high capacitance is employed, and the capacitor can be provided by forming a layer of high dielectric constant on the wiring board. In the case where the frequency range employed for wireless communication is 300 MHz or more, high antenna performance is attained through a known method employing a wavelength-shortening effect (i.e., the size of an antenna is almost inversely proportional to the square root of dielectric constant).
In recent years, with development of high-density electronic devices and widespread use of wireless data communication devices (e.g., cellular phones), demand has arisen for absorption of unnecessary radio waves.
Hitherto, a resin composite material containing, as a filler, a large amount of powder of ferrite or a soft-magnetic alloy has been employed for such radio wave absorption (see, for example, Patent Document 1). However, with an increase in radio frequency used (toward a microwave range), such a resin composite material raises problems in terms of reduction of magnetic permeability, along with an increase in thickness required for attaining absorption property. In addition, such a resin composite material, which contains, as a filler, a large amount of powder of high specific weight, poses a problem in that the composite material is not suitable for reducing the weight of, in particular, a portable communication device, because of high specific weight of the composite material.
Meanwhile, there has been proposed a method for dispersing, in an insulating medium, a typical electrically conductive filler, such as particles of a carbon material (e.g., graphite or carbon black), or so-called electrically conductive titanium oxide (i.e., titanium oxide coated with antimony-doped tin oxide) (see, for example, Patent Document 2). However, such a method raises a problem in that when the amount of an electrically conductive filler employed is increased so as to reduce the thickness of a radio wave absorber (radio-wave-absorbing sheet) to 1/20 or less the wavelength of radio waves employed (i.e., to increase dielectric constant), a deviation occurs from non-reflective conditions represented by the below-described formula (i.e., in the case where the radio wave absorber is sufficiently separated from a data signal source). Particularly when dielectric constant is to be increased to 20 or more, a considerable deviation occurs, and thus the radio-wave-absorbing sheet must be thickened. Alternatively, when the radio-wave-absorbing sheet is thinned, limitations are imposed on the sheet; for example, the sheet is applied only to radio waves having a wavelength of 1 cm or less and a frequency of 30 GHz or more.
                              ɛ                =                  tanh          (                      2            ⁢            π            ⁢                                                  ⁢            ⅈ            ⁢                          ɛ                        ⁢                          d              λ                                )                                    [        F1        ]            (wherein ∈: complex relative permittivity, d: thickness of radio wave absorber, λ: wavelength of radio wave, i: imaginary unit)
Conventionally, there has been proposed a high-dielectric-constant resin composite material which contains, as a filler, a ferroelectric material (e.g., barium titanate) serving as a high-dielectric-constant filler, in an amount of 65 vol. % or more (i.e., 80 wt. % or more) (see, for example, Patent Document 3).
Meanwhile, there has been proposed a high-dielectric-constant composition prepared through insulation coating of electrically conductive powder with a thermosetting resin (see, for example, Patent Document 4). However, the composition fails to exhibit reliable performance, and thus is not produced on a commercial scale. In practice, a large amount of the aforementioned filler is added to the composition. Therefore, although high dielectric constant is attained, fundamental characteristics of resin material (i.e., workability, moldability, and light weight) are impaired.
A high-dielectric-constant material containing a large amount of an inorganic filler has not yet been commercially employed in, particularly, an antenna board for a non-contact IC card.
For example, in the case of a non-contact IC card employing a frequency of 13.56 MHz, a loop-like wiring pattern serving as a coil is formed on a generally employed resin substrate having a relative dielectric constant of 5 or less, and no capacitor is provided on the substrate. In this IC card, the loop-like wiring pattern serves as an aperiodic magnetic pickup coil, and communication distance is reduced to 10 cm even in an ideal case (in practice, to 1 cm or less).
Since the dielectric constant of the resin constituting the substrate is generally 5 or less, a large electrode area is required for forming a tuning capacitor. In view of the foregoing, there has been proposed a technique in which a plurality of electrode patterns are formed on an antenna board so as to secure a predetermined electrode area, and then the resultant substrate is folded, followed by connection of the electrodes by through-hole wiring (see, for example, Patent Document 5); as well as a technique in which an electrode area required for a tuning capacitor is reduced by increasing the size of an antenna coil (see, for example, Patent Document 6). In the case of the former technique, the antenna board has a complicated structure, and, due to electromagnetic induction between capacitor electrodes formed at a center portion of an antenna coil, magnetic flux is considerably reduced in the antenna coil, and thus sensitivity is reduced. In the case of the latter technique, the size of an antenna board itself is increased. Therefore, in practice, most commercial devices employ a magnetic pickup coil.
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2003-327831
Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2002-57485
Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. 2001-237507
Patent Document 4: Japanese Patent Application Laid-Open (kokai) No. S54-11580
Patent Document 5: Japanese Patent Application Laid-Open (kokai) No. 2002-358479
Patent Document 6: Japanese Patent Application Laid-Open (kokai) No. 2002-183689